1. Field of the Invention
The present invention relates to a process and a device for forming calcium oxide from calcium carbonate.
It finds a particularly important, though not exclusive, application in the field of the manufacture of calcium oxide of high purity, that is to say in which the percentage by weight of impurities relative to the total weight is lower than a few ppm, by electrical heating of pulverulent calcium carbonate, that is to say of a mean particle size smaller than of the order of 50 .mu.m, obtained from limestone rock.
2. State of the Art
The production of calcium oxide by heating calcium carbonate and according to the reaction CaCO.sub.3 .fwdarw.CaO+CO.sub.2 is a process which is very well known.
The chemical purity of the calcium oxide obtained depends directly, on the one hand, on the initial purity of the calcium carbonate employed and hence a priori on the mineralogical qualities of the limestone quarry from which it originates, and, on the other hand, on the calcination oven employed.
Calcination ovens which are traditionally employed are, in fact, a source of contamination because they are generally lined internally with aluminous or magnesium-containing refractory materials which remain in contact with the products during the chemical reaction. In addition, the heating is frequently produced by a combustion flame of a gas or other fuel product, generating fumes which also contaminate the calcium oxide formed.
While the processes and devices for forming calcium oxide employing calcination ovens of this type are sufficient for producing so-called commercial lime, where the impurities represent a few percent by weight, they are clearly insufficient for producing on an industrial scale pure and reactive limes where the impurities do not represent more than a few ppm.
It is an object of the invention to provide an improved process and device for forming calcium oxide. It is a more specific object to provide a process for forming very pure and very reactive calcium oxide continuously and on an industrial scale, by electrically heating calcium carbonate as a fluidised bed; this makes it possible to have little contamination both in the case of the calcium oxide formed and of the environment, while retaining a complete control over the temperature parameters.
Electrical heating of a pulverulent material as a fluidised bed in a furnace or reactor is known. However, this heating process has not hitherto been applied to the formation of calcium oxide, especially on an industrial scale, from pulverulent calcium carbonate. In fact, between 620.degree. C. and 750.degree. C., pulverulent calcium carbonate exhibits a softening region where the particles tend to agglomerate together. This softening stage, sometimes called "pseudosintering stage" by some authors, which is proportionally more pronounced the smaller the particle size, does not therefore a priori allow a fluidised bed of pulverulent calcium carbonate to be heated to a sufficient temperature for the dissociation to calcium oxide to take place.
The invention allows this disadvantage to be overcome.
To this end there is provided a process for forming calcium oxide from pulverulent calcium carbonate, comprising the steps of:
preparing an initial bath of pulverulent calcium oxide within a reactor, said initial bath being in the form of a fluidised bed heated electrically to a temperature higher than of the order of 900.degree. C.,
and continuously introducing the pulverulent calcium carbonate into said initial bath, which forms a dissociating bath, while maintaining the temperature of said dissociating bath at a temperature higher than of the order of 860.degree. C. by electrical heating, whereby said calcium carbonate is dissociated into calcium oxide and carbon dioxide, within said dissociating bath.
Generally, in order to keep the volume of the bath forming the fluidised bed substantially constant, a continuous extraction of calcium oxide from the reactor, for instance at an opposite side from the introduction of calcium carbonate, is simultaneously provided. This allows a continuous process for forming calcium oxide.
The invention has parts of its origin in the following observations:
The softening stage does not exist in the case of pulverulent calcium oxide, which can therefore be heated as a fluidised bed to 900.degree. C.
It was furthermore found experimentally that the continuous introduction of "cold" pulverulent calcium carbonate, while heating to keep the temperature at all parts of the bath above the threshold temperature for decarbonation, made it possible to dispense with the risks of agglomeration of the particles of pulverulent calcium carbonate preventing fluidisation from continuing.
An other object of the invention is to provide a process wherein the initial bath of pulverulent calcium oxide is in the form of a fluidised bed obtained gradually from calcium carbonate or limestone whose particle size is reduced in stages. It has been observed by the inventors that the softening stage of calcium carbonate is a function of the particle size of the heated powder and that it almost disappears completely in the case of a particle size larger than 300 .mu.m.
In such an embodiment as described above, a first bath in the form of a fluidised bed consisting of limestone of mean particle diameter greater than 300 .mu.m, for example of approximately 350 .mu.m, is heated to a temperature above approximately 900.degree. C. and then, while the temperature of the fluidised bed is maintained above 900.degree. C., limestone of a mean particle diameter of between 150 and 80 .mu.m is introduced gradually while the fluidisation flow rates are adjusted to obtain a bed of quicklime of the same particle size and, finally, pulverulent calcium carbonate, for example of mean particle diameter smaller than of the order of 40 .mu.m is introduced gradually until the said initial bath of calcium oxide is obtained in the form of a fluidised bed at a temperature higher than of the order of 900.degree. C.
It is another object of the invention to provide a process wherein the calcium carbonate is obtained by calcining a precrushed limestone rock, separating the calcinate and the carbon dioxide from dissociation, hydrating the calcinate to remove impurities, and recombining the calcinate with recovered carbon dioxide from dissocation to form pulverulent calcium carbonate in precipitated form.
Since the calcium carbonate thus obtained is particularly pure, the quality of the calcium oxide produced by thermal dissociation of such a carbonate is further improved thereby.
In addition, in other advantageous embodiments, either or both of the following arrangements are resorted to:
the bath in the form of a fluidised bed is heated by a network of electrical Joule effect heating elements which are distributed and arranged directly within the fluidisation bath,
air is employed as fluidising gas.
It is a further object of the invention to provide a device for making use of the process according to the invention, characterised in that it comprises a reactor having a vessel defining a fluidisation chamber whose inner walls are metal sheets made of or covered with refractory metals or metal alloys.
An other object of the invention is to provide a Device for forming calcium oxide from pulverulent calcium carbonate comprising:
a reactor having a vessel defining a fluidisation chamber whose inner walls are metal sheets made of or covered with refractory metals or metal alloys,
means for fluidising pulverulent calcium oxide and pulverulent calcium carbonate within said chamber, in the form of a fluidised bed,
means for heating electrically to a temperature higher than of the order of 900.degree. C. said fluidised bed,
means for continuously introducing the pulverulent calcium carbonate into said fluidised bed, while maintaining the temperature of said fluidised bed at a temperature higher than of the order of 860.degree. C. by electrical heating, whereby said calcium carbonate is dissociated into calcium oxide and carbon dioxide, within said fluidised bed, and:
means for continuously extracting the pulverulent calcium oxide from said reactor.
In an advantageous embodiment the reactor is provided with means for feeding calcium carbonate into the lower part of the fluidised bed and with means for removing calcium oxide in the upper part of the fluidised bed.
The invention will be understood better on reading the particular embodiments which are given by way of examples, no limitation being implied.